In the present study, theoretical results of the heat transfer characteristics and the performance of the annular fin under partially wet surface conditions are presented. The mathematical models based on the conservation equations of mass and energy are developed. The central finite difference method is employed for solving this model to obtain temperature distribution of tube wall and fin. Effects of inside Biot number, outside Biot number, relative humidity of air, and hot fluid temperature on the heat transfer characteristics of annular fin are considered. The temperature distribution are presented in the dimensionless form, (T − Th ) (Tc − Th ) . It is found that the inlet of both working fluids conditions have significant effect on the heat transfer characteristics of annular fin. In addition, the position of wet-dry interface is affected by the working fluids conditions and the fin dimensions.